1. Field of the Invention
The present invention relates to a ceramide-containing lipid composition and a liquid crystal formulation, in which the lipid composition is dispersed stably, and more particularly to a skin care formulation, skin-protecting agent, bath agent and hair care formulation containing the same.
2. Description of the Related Art
A skin plays a very important role as a barrier film which prevents any loss of biologically essential components such as water while serving for a protection from any biological, chemical or physical invasion of external microorganisms, chemicals, ultraviolet light and the like. The part functioning as a barrier film is a horny cell layer whose thickness is about 20 μm and which is located in the outermost layer of the skin and forms a tough barrier film in a structure of the corneocyte stacked as bricks binding to each other via intercellular lipids serving as a mortar. A ceramide is known to play an important role for keeping a skin soft and moist by constructing a lipid barrier as a key component in an inter-corneocyte lipid (see Downing D. T., et al., J. Lipid. Res., 24, 759(1983), Downing D. T., et al., J. Invest. Dermat., 84, 410 (1985)). A ceramide is classified into any of types 1 to 7 based on the structures of a sphingosine base and an acyl side chain as its constituents, and a naturally occurring sphingosine base is known to have a steric structure which is a D-erythro form as an optically active substance.
Recently, a rough skin, dry skin and atopic dermatitis skin are known to have an extremely reduced ceramide content in the inter-corneocyte lipid described above when compared with that in a normal healthy skin, and an attempt is made to improve the roughness of a skin by supplementing a ceramide to a dry or rough skin.
However, any of the ceramides is a hardly soluble compound which has a high crystallinity and a high melting point, and its unique amphiphillic structure leads to an extremely low solubility in almost all oil-soluble/water-soluble cosmetic base material (solvent), resulting in a problem that is difficult in formulating into a product. Thus, when a large amount of ceramide is contained in a product, it undergoes a crystal precipitation, which affects the safety of the product, or becomes less safe especially when combined with a certain oily material for solubilizing the ceramide. Such a problematic tendency becomes more evident when handling a naturally occurring type optically active ceramide, and is desired to be solved.
In a prior attempt (1), a liquid crystal lipid composition consisting of an optically active ceramide and a sterol is proposed to be utilized in a cosmetic product or a skin care formulation (See Japanese Patent Application Laid-Open (JP-A) Nos. 11-12118 and 2001-348320). By using such a liquid crystal composition, an excellent water barrier performance and moistening performance of a naturally occurring type optically active ceramide is realized, but such a liquid crystal lipid has a melting point of 105° C. or more, because of which a special device or a complicated process for formulating a product is required and a further improvement in the stability of the formulated product over a prolonged period is also required.
On the other hand, another prior attempt (2) involves a reduction in the melting point of a ceramide by using a ceramide type 3 having a steric isomeric structure identical to that of a naturally occurring one in combination with a ceramide type 5 and/or a ceramide type 2 (see JP-A 8-225427). In this attempt (2), the ceramide which is available as a steric isomer identical to a naturally occurring one and capable of being in a lamellar structure is only the ceramide type 3, and other naturally occurring type ceramides are taught to be difficult to obtain, and the ceramide types 2 and 5 are employed as racemic mixtures. Thus, the reduction in the melting point is proposed to accomplish by adding the ceramide type 2 which is a mixture of four isomers and the ceramide type 5 which is a mixture of 8 isomers.
While a racemic ceramide is known to have a lower melting point because it is a mixture of isomers, it is also known to have an extremely low effect when compared with a naturally occurring type ceramide, and the reduction in the melting point by adding the racemic ceramide leads to a problem with a reduction also in the efficacy.